Associated with the information revolution is a need to increase by many orders of magnitude the rate of information transfer. This revolution is enabled by the switch from copper wire to optical fiber. Efficient implementation of this change requires optical switches to move data from one fiber to another. For a large number of input and a large number of output fibers, this switch is typically referred to as an optical crossbar switch.
A typical component of an optical crossbar switch is a fiber array coupled to a microlens array in such a way that an array of substantially collimated and parallel beams leave the assembly. A schematic of a microlens array is illustrated in FIG. 1. Each microlens array 100 is comprised of a plurality of lenslets 110. In the typical case, each optical fiber is associated with a single lenslet 110.
A one-to-one mapping exists between fibers and optical beams leaving the assembly. The system performance is enhanced if each optical beam is substantially focused on the end of its respective optical fiber. The construction of such a system is simplified if all of the beams focus through the microlens array at substantially the same distance. In such a case, the ends of all the optical fibers are arranged on a plane that is a uniform distance from the microlens array. This requires that the microlens array have a high degree of uniformity with respect to the distance at which each lenslet focuses.
Manufacturing a microlens array with sufficiently high uniformity with respect to the focus distance is expensive. Most often, the problem is associated with variations in the focal length of the individual lenslets. However, for the purposes of this patent document, variations in the focus or focus distance can be due to focal-length variations of the lenslets or any other source of nonuniformity. In more affordable microlens arrays the focus distance tends to vary slowly across the array. A typical variation is illustrated in FIG. 2. For this particular microlens array, the low regions 120 indicate portions of the microlens array for which the focus distance is as much as 3% less than the nominal value. The high regions 130 indicate portions of the microlens array for which the focus distance is as much as 4% greater than the nominal value.
To reduce the cost of an optical crossbar switch and maintain satisfactory performance, a means for compensating for the microlens focus variations needs to be developed.